hEGF is a polypeptide hormone consisting of 53 amino acids secreted mainly from the duodenum and sub maxillary glands. hEGF is capable of restraining gastric acid secretion and expediting the growth of epidermal cells. The hEGF function in restraining gastric acid indicates its possible use as a medicine for treating duodenal ulcers. Besides, hEGF is known to cause various biological reactions as a result of its combination with the EGF receptor located at the membranous surface of cells [D. Gospodarowicz, Ann. Rev. Physiol., 43, 251 (1981)]. Reaction caused by EGF is identical to that caused by the product of oncogene of tumor virus. It is therefore interesting from the view point of searching for the mechanism of carcinogenesis to elucidate the role of EGF in vivo and the mechanism of developing and controlling cells. However, since natural hEGF exists in extremely small in quantities, the production of hEGF by means of recombinant DNA has begun to attract a great deal of attention. Obtaining the hEGF gene from human tissues is difficult due to the presence of numerous restriction sites. In fact, the DNA sequence of hEGF has not yet been determined.
One example for the expression of a chemically synthesized structural gene based on the known amino acid sequence of hEGF has been reported [H. Gregory, Nature, 257, 325 (1975)]. However, the EGF is expressed only as a fusion peptide, and considering that it is a comparatively low molecular weight peptide, it is recognized as a foreign substance in a bacteria and easily digested with enzyme [J. Smith et al., Nucleic Acids Research, 10, 4467 (1982)]. A method of removing unnecessary parts from the fusion peptide has been proposed, but it is extremely uncertain (Stephen James Bruer et al., EP Patent Provisional Publication No. 89 626). An example of direct expression of the hEGF gene itself was conducted in yeast [M. S. Urdea et al., Proc. Natl. Acad. Sci. U.S.A., 80, 7461 (1983)]. The quantity of the expression was so little, and coupled with the slow yeast growth, the method is not adequate for quantity production.